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The Object#

A controller always needs a source of truth for what the world should look like, and this object always lives inside kubernetes.

Depending on how the object was created/imported or performance optimization reasons, you can pick one of the following object archetypes:

  • typed Kubernetes native resource
  • Derived Custom Resource for Kubernetes
  • Imported Custom Resource already in Kubernetes
  • untyped Kubernetes resource
  • partially typed Kubernetes resource

We will outline how they interact with controllers and the basics of how to set them up.

Typed Resource#

This is the most common, and simplest case. Your source of truth is an existing Kubernetes object found in the openapi spec.

To use a typed Kubernetes resource as a source of truth in a Controller, import it from k8s-openapi, and create an Api from it, then pass it to the Controller.

use k8s_openapi::api::core::v1::Pod;

let pods = Api::<Pod>::all(client);
Controller::new(pods, watcher::Config::default())

This is the simplest flow and works right out of the box because the openapi implementation ensures we have all the api information via the Resource traits.

If you have a native Kubernetes type, you generally want to start with k8s-openapi. If will likely do exactly what you want without further issues. That said, if both your clusters and your chosen object are large, then you can consider optimizing further by changing to a partially typed resource for smaller memory profile.

A separate k8s-pb repository for our future protobuf serialization structs also exists, and while it will slot into this category and should hotswappable with k8s-openapi, it is not yet usable here.

Custom Resources#

Derived Custom Resource#

The operator use case is heavily based on you writing your own struct, and a schema, and extending the kuberntes api with it.

This has historically required a lot of boilerplate for both the api information and the (now required) schema, but this is a lot simpler with kube thanks to the CustomResource derive proc_macro.

/// Our Document custom resource spec
#[derive(CustomResource, Deserialize, Serialize, Clone, Debug, JsonSchema)]
#[kube(kind = "Document", group = "", version = "v1", namespaced)]
#[kube(status = "DocumentStatus")]
pub struct DocumentSpec {
    name: String,
    author: String,

#[derive(Deserialize, Serialize, Clone, Debug, JsonSchema)]
pub struct DocumentStatus {
    checksum: String,
    last_updated: Option<DateTime<Utc>>,

This will generate a pub struct Document in this scope which implements Resource. In other words, to use it with the a controller is at this point analogous to a fully typed resource:

let docs = Api::<Document>::all(client);
Controller::new(docs, watcher::Config::default())

Custom resources require schemas

Since v1 of CustomResourceDefinition became the main variant (v1beta1 was removed in Kubernetes 1.22), a schema is required. These schemas are generated using schemars by specifying the JsonSchema derive. See the schemas section (TODO) for further information on advanced usage.


Before Kubernetes accepts api calls for a custom resource, we need to install it. This is the usual pattern for creating the yaml definition:

# Cargo.toml
name = "crdgen"
path = "src/"
use kube::CustomResourceExt;
fn main() {
    print!("{}", serde_yaml::to_string(&mylib::Document::crd()).unwrap())

Here, a separate crdgen bin entry would install your custom resource using cargo run --bin crdgen | kubectl -f -.

Installation outside the controller

While it is tempting to install a custom resource within your controller at startup, this is not advisable. The permissions needed to write to the cluster-level customresourcedefinition resource is almost always much higher than what your controller needs to run. It is thus advisable to generate the yaml out-of-band, and bundle it with the rest of the controller's installation yaml.

Imported Custom Resource#

In the case that a customresourcedefinition already exists in your cluster, but it was implemented in another language, then we can generate structs from the schema using kopium.

Suppose you want to write some extra controller or replace the native controller for PrometheusRule:

curl -sSL \
    | kopium -Af - >

this will read the crd from the cluster, and generate rust-optimized structs for it:

use kube::CustomResource;
use schemars::JsonSchema;
use serde::{Serialize, Deserialize};
use std::collections::BTreeMap;
use k8s_openapi::apimachinery::pkg::util::intstr::IntOrString;

/// Specification of desired alerting rule definitions for Prometheus.
#[derive(CustomResource, Serialize, Deserialize, Clone, Debug, JsonSchema)]
#[kube(group = "", version = "v1", kind = "PrometheusRule", plural = "prometheusrules")]
pub struct PrometheusRuleSpec {
    /// Content of Prometheus rule file
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub groups: Option<Vec<PrometheusRuleGroups>>,

/// RuleGroup is a list of sequentially evaluated recording and alerting rules.
#[derive(Serialize, Deserialize, Clone, Debug, JsonSchema)]
pub struct PrometheusRuleGroups {
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub interval: Option<String>,
    pub name: String,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub partial_response_strategy: Option<String>,
    pub rules: Vec<PrometheusRuleGroupsRules>,

/// Rule describes an alerting or recording rule
#[derive(Serialize, Deserialize, Clone, Debug, JsonSchema)]
pub struct PrometheusRuleGroupsRules {
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub alert: Option<String>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub annotations: Option<BTreeMap<String, String>>,
    pub expr: IntOrString,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub r#for: Option<String>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub labels: Option<BTreeMap<String, String>>,
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub record: Option<String>,

you typically would then import this file as a module and use it as follows:

use prometheusrule::PrometheusRule;

let prs: Api<PrometheusRule> = Api::default_namespaced(client);
Controller::new(prs, watcher::Config::default())

Kopium is unstable

Kopium is a relatively new project and it is neither feature complete nor bug free at the moment. While feedback has been very positive, and people have so far contributed fixes for several major customresources; expect some snags.

Dynamic Typing#

Untyped Resources#

Untyped resources are using DynamicObject; an umbrella container for arbitrary Kubernetes resources.

Hard to use with controllers

This type is the most unergonomic variant available. You will have to operate on untyped json to grab data out of specifications and is best suited for general (non-controller) cases where you need to look at common metadata properties from ObjectMeta like labels and annotations across different object types.

The DynamicObject consists of just the unavoidable properties like apiVersion, kind, and metadata, whereas the entire spec is loaded onto an arbitrary serde_json::Value via flattening.

The benefits you get is that:

  • you avoid having to write out fields manually
  • you can achieve tolerance against multiple versions of your object
  • it is compatible with api discovery

but you do have to find out where the object lives on the api (its ApiResource) manually:

use kube::{api::{Api, DynamicObject}, discovery};

// Discover most stable version variant of ``
let apigroup = discovery::group(&client, "").await?;
let (ar, caps) = apigroup.recommended_kind("Document").unwrap();

// Use the discovered kind in an Api, and Controller with the ApiResource as its DynamicType
let api: Api<DynamicObject> = Api::all_with(client, &ar);
Controller::new_with(api, watcher::Config::default(), &ar)

Other ways of doing discovery are also available. We are highlighting recommended_kind in particular here because it can be used to achieve version agnosticity.

Multiple versions of an object

Kubernetes supports specifying multiple versions of a specification, and using DynamicObject above can help solve that. There are other potential ways of achieving similar results, but it does require some work.

Partially-typed Resource#

A very special-case setup where we specify a subset of the normal typed information, and allows tighter control over memory characteristics, and deserialization cost of the program, but at the cost of more struct code.

Better methods available for improving memory characteristics

Because almost all methods on Kubernetes objects such as PodSpec are wrapped in Options, as long as unnecessary properties are unset before passing them to a reflector, similar memory reductions can be achieved. One method is to use Event::modify chained onto the watcher stream. See the pod_reflector for details.

Because of these advances, the partially-typed resource pattern is not recommended.

It is similar to DynamicObject (above) in that Object is another umbrella container for arbitrary Kubernetes resources, and also requires you to discover or hard-code an ApiResource for extra type information to be queriable.

Here is an example of handwriting a new implementation of Pod by overriding its spec and status and placing it inside Object, then stealing its type information:

use kube::api::{Api, ApiResource, NotUsed, Object};

// Here we replace heavy type k8s_openapi::api::core::v1::PodSpec with
#[derive(Clone, Deserialize, Debug)]
struct PodSpecSimple {
    containers: Vec<ContainerSimple>,
#[derive(Clone, Deserialize, Debug)]
struct ContainerSimple {
    image: String,
// Pod replacement
type PodSimple = Object<PodSpecSimple, NotUsed>;

// steal api resource information from k8s-openapi
let ar = ApiResource::erase::<k8s_openapi::api::core::v1::Pod>(&());

Controller::new_with(api, watcher::Config::default(), &ar)

In the end, we end up with some extra lines to define our Pod, but we also drop every field inside spec + status except spec.container.image. If your cluster has thousands of pods and you want to do some kind of common operation on a small subset of fields, then this can give a very quick win in terms of memory use (a Controller will usually maintain a Store of all owned objects).

Dynamic new_with constructors#

Partial or dynamic typing always needs additional type information

All usage of DynamicObject or Object require the use of alternate constructors for multiple interfaces such as Api and Controller. These constructors have an additional _with suffix to carry an associated type for the Resource trait.


All the fully typed methods all have a consistent usage pattern once the types have been generated. The dynamic and partial objects have more niche use cases and require a little more work such as alternate constructors.

typing Source Implementation
full k8s-openapi use k8s-openapi::X
full kube::CustomResource #[derive(CustomResource)]
full kopium kopium >
partial kube::core::Object partial copy-paste
none kube::core::DynamicObject write nothing